Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an unaffected father, XDY. The mother's gametes run down the side. Click the cell for an unaffected son.
♀ XDXd♂ XDY
- 1Xd × XD
- 2Xd × Y
- 3XD × XD
- 4XD × Y
2.GeneticsHard
A recessive condition affects 1 in 2500 of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.0004
f =
3.Cell biologyMedium
Where in the cell does the citric acid (Krebs) cycle take place?
- AInner mitochondrial membrane
- BMitochondrial matrix
- CIntermembrane space
- DCytoplasm
4.Cell biologyHard
A disease is passed from an affected mother to all her children but never from an affected father. Where is the mutation most likely located?
- AIn mitochondrial DNA
- BOn the X chromosome, as a dominant trait
- COn an autosome, as a recessive trait
- DOn the Y chromosome
3Solutions
1.Genetics
Answer: XD × Y
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Fill the square
XdXD, XdY, XDXD, XDY
Find the cell
XD × Y = XDY
Answer
Cell XD × Y → XDY
2.Genetics
Answer: f = 0.0392
Hardy–Weinberg: in a large, randomly mating population with no selection, the allele frequencies p (dominant) and q (recessive), p + q = 1, give genotype frequencies p², 2pq and q².
Allele frequencies
q = √0.0004 = 0.02
p = 1 − 0.02 = 0.98
Carriers
2pq = 2·0.98·0.02 = 0.0392
Answer
f = 0.0392
3.Cell biology
Answer: B · Mitochondrial matrix
Why this is the answer
The citric acid cycle runs in the mitochondrial matrix, producing NADH, FADH₂ and CO₂. The electron transport chain sits in the inner membrane.
Answer
B · Mitochondrial matrix
4.Cell biology
Answer: A · In mitochondrial DNA
Why this is the answer
Mitochondria, and their DNA, are inherited almost only from the egg. Mitochondrial disorders therefore follow maternal inheritance.
Answer
A · In mitochondrial DNA
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